Pathos Classic Mk 3 Problem, Please Help

Pathos Classic MkIII left channel goes dead, just a very light hum, after ten mins of use on all inputs, then powers back up fine after restart for 10 more mins. Unit worked fine for me then took a 2000 mile road trip boxed in the back seat of a car to the new owner's house. This never happened before. Any ideas would be greatly appreciated. Thanks.
A 2000 mile road trip is not only hard on the human body but also anything else along for the ride. I would have to guess that something is a ajar like a soldering joint or just loose. Try looking at the rca jacks and also check the speaker lugs? And if not look inside and check for anything that looks out of the ordinary and if all else fails time to ship it back and have a tech look at it.The best with it.
First order of business, check connections (sorry if offensive, but weirder things have happened; I had a cable which went bad that mimicked problems with an amp).

Second, get a replacement tube. I'm not an expert on tube construction, but it is possible in my thinking that an element of the tube expands upon heating up to a certain point and the contact is lost. Swap the two tubes and see if the problem moves to the other channel. If so, it's a tube issue. Be cautious and purposeful moving the tubes; get the pins lined up and push down carefully so as not to bend or break them. You can do it, just take your time.

Swapping the tubes should not hurt the unit; operate it in low volume as you are testing and have a finger on the mute button in case. These are extreme precautions which are likely not necessary, but I always take extra precautions when diagnosing and changing things with amps. If you have not yet exchanged tubes I would guess it might be the tube gone kaput.

Have you conducted tests to make sure it's not the source?

If none of this resolves the issue it's possible there is a solder joint which is opening up when the unit heats up. A tech should be able to find it and fix it fairly easily.

If still a problem contact Pathos to find a tech nearby. You an email Gianni Borinato at Pathos - he's the designer of the MkIII. If anyone on earth has an idea of it, it's him. Send the email to pathos' headqarters with his name and he'll get it.
Thanks for your input. Pun intended.
Thanks to your advice, when the new owner switched the tubes around he discovered that one tube was bad. We are wondering what a suitable, not used, new replacement tube might be right for this application. Thanks again.
The stock tubes (I think they're Sovtek) are excellent sounding. Just get replacements of the same thing. Tube sellers like Upscale Audio should have them. Buy a pair and put the new pair in. You'll have one for spare.

I think in one unit (MkII I believe) I tried a different set, maybe Electro Harmonix. I ended up going back to the stock tubes as they sounded just as good. It's possible an older, more rare tube - Maybe a Mullard or similar - would sound better, but it'll probably cost quite a bit more.
The Russian made Voskhod 6H23 Rocket Logo is the best tube for this amp period. Friend of mine got these for his Pathos and flipped out at the improvement.